In most communication systems, and in particular wireless communication systems, it is of outmost importance to provide a reliable protocol for delivering data units from one entity to at least another entity in the system, without loss of data and without duplication of data. Such reliable data delivery protocols typically rely on the principle that the receiver of the data responds to the sender of the data with acknowledgements upon reception of the data and/or negative acknowledgements if the data units were lost. The sender will subsequently to the acknowledgement send the next data unit, or in the vent of a negative acknowledgement, retransmit the lost data unit.
Automatic repeat request (ARQ) is one of the most common retransmission techniques in communication networks, and ensures reliable user data transfer and data sequence integrity. The data is, prior to the transmission, divided into smaller packets, protocol data units (PDU). A reliable transfer is enabled by encoding packets with an error detecting code, such that the receiver can detect erroneous or lost packets and thereby order retransmission. The data sequence integrity is normally accomplished by sequential numbering of packets and applying certain transmission rules.
In the simplest form of ARQ, commonly referred to as Stop-and-Wait ARQ, the sender of data stores each sent data packet and waits for an acknowledgement from the receiver of a correctly received data packet, by way of an acknowledgement message (ACK). When the ACK is received, the sender discards the stored packet and sends the next packet. An example of a prior art Stop-and-Wait ARQ scheme is schematically depicted in the message sequence chart of FIG. 1a. The process is typically supplemented with timers and the use of negative acknowledgement messages (NACK), which is illustrated in FIG. 1b. The sending entity uses a timer, which is started on the transmission of a data packet, and if no ACK has been received before the timer expires the data packet is retransmitted. If the receiver detects errors in the packet it can send a NACK to the sender. Upon receiving the NACK, the sender retransmits the data packet without waiting for the timer to expire. If the ACK or NACK message is lost, the timer will eventually expire and the sender will retransmit the data packet. From the simple Stop-and-Wait, more elaborated schemes of the conventional ARQ has been developed, for example Go-Back-N and Selective Reject (or Selective Repeat), which provides a higher throughput. Taught in WO 02/09342 by Dahlman et al. is a ARQ scheme that adds flexibility to the traditional ARQ scheme by introducing ARQ parameters that are set and/or negotiated to give a desired benefit in regard to communication resources.
In another line of development of the ARQ, the redundancy in the coding is exploited in various ways to enhance communication performance (generally measured as throughput). These schemes are referred to as Hybrid ARQ schemes. The fundamentals of Hybrid ARQ are described in “Error Control Coding: Fundamentals and Applications” by Shu Lin and Daniel J. Costello, Prentice Hall, 1983, pp 477-494. Due to the combination of coding and ARQ, the hybrid ARQ schemes can give a certain adaptation to changes in the radio environment, e.g. to fading. How to best combine ARQ and coding schemes to cope with fading channels is not trivial. Several approaches and schemes have been suggested and used.
Recent developments of the HARQ protocols include Chase combining and incremental redundancy approaches, and are described in “Performance comparison of HARQ with Chase combining and incremental redundancy for HSDPA” by Frenger, P., Parkvall, S., Dahlman, E., VTC 2001 Fall. IEEE VTS 54th, Volume: 3, 7-11, October 2001, pp 1829-1833 vol. 3. In the Chase combining scheme, HARQ-CC, also referred to as Type I HARQ, the same bit sequence as that used in the initial transmission is also used for the retransmission, and the receiver always combines the full retransmission of the failed block. Note that the amount of buffered coded bits in the receiver buffer remains the same. In the incremental redundancy schemes HARQ-IR, also referred to as type II HARQ, the retransmission introduces new coded bits to the previous transmitted blocks. That is, the amount of data to be buffered in the receiver increases with consecutive retransmissions. Two alternative schemes are considered for the incremental redundancy, systematic priority and parity priority.
It is in the art recognised that HARQ schemes typically are sensitive to changes and fluctuations in the radio environment, for example the fluctuations in a channel as regards to signal strength due to Rayleigh fading or unpredictable interference variations. The channel fluctuation may further cause inaccuracies in channel measurements and/or that outdated channel measurements are used for link mode selection. This may cause packets to be sent with a rate that is not decodable if the interference and noise is greater than permitted for the selected rate. Alternatively, a margin may be introduced and a reduced rate used, but this is done at the “cost” of not efficiently using the channels that can bear a higher rate. U.S. Pat. No. 6,101,168 describes a method and arrangement providing some adaptation to changes in the channel by symbol accumulation in the receiver. The data not correctly received is retransmitted concurrently with new data, but at a different code rate and/or at a different power levels.
Wireless communication systems utilizing a plurality of components for the transmission of a single message, for example Orthogonal Frequency Division Multiplexing (OFDM) systems, put additional requirements on a retransmission scheme. In such system, a forward error correction (FEC) codeword is carried by multiple components of varying channel quality states. Therefore, several difficulties in maintaining reliable retransmission are present:                Multi-state channel quality variation causes an inherent loss in the transmission. A codeword over a multi-state channel with a high degree of variations requires higher operating signal to noise ratio, SNR, for the same target quality (block error rate, BLER, for example) as compared with a multi-state channel with a lower degree of variations.        For components, or channel states, with low quality, channel estimation is more prone to errors. With bad channel estimates, the inclusion of low-quality components effectively subtracts energy from better-quality components.        For turbo codes, wide channel quality variation could significantly erase systematic bits. Without enough systematic bits the turbo decoder fails.        
By not taking the above difficulties into account the prior art HARQ protocols exhibit the drawbacks:                HARQ-CC always retransmits identical coded bits;        HARQ-IR with parity priority always retransmits new coded bits of the same size. Furthermore, it does not correct systematic bit erasure problems;        HARQ-IR with systematic priority retransmits the complete systematic bits and some new coded bits to fill up the size.        
Consequently, wasteful use of valuable radio resources cannot be avoided. In addition, the prior art HARQ protocols do not focus on the problem caused by the multi-state channel.